Thermoplastic compositions are often used in the manufacture of products requiring the joining of separate previously-formed articles, such as through laser-welding. Near-infrared (NIR) laser-welding of two polymer articles by transmission welding requires one of the polymer articles to be at least partially transparent to NIR laser light, and the other to absorb a significant amount of the NIR laser light. The laser passes through the first laser transparent layer and is absorbed by the second polymer layer, generating heat in the exposed area. External pressure is applied to ensure uninterrupted contact and heat conduction between the parts resulting in the melting of both the absorbing and the transmitting polymers, thus generating a weld at the interface.
The level of NIR transmission in the upper part should allow sufficient laser density at the interface to facilitate effective welding. Otherwise, the joining of the two materials by laser transmission welding is either difficult or restricted to slow scan speeds, which undesirably lengthens the part assembly cycle time. Partially crystalline polyesters, such as poly(butylene terephthalate), are materials that can easily disperse the incoming radiation through a combination of back scattering and internal diffusion, thereby causing unwanted broadening of the NIR laser beam. Consequently, the laser energy at the joining interface is diminished and the adhesion between the two layers is reduced. Scattering effects are greatly enhanced when fillers such as glass fibers are present, especially when the upper layer thickness is greater than 1 millimeter. Additionally, the internal scattering of the laser in the first (upper) part can bring about a rise in temperature, especially in thick walled parts. Coupled with the fact that the crystallization pattern across a complex part varies, it is therefore beneficial to have high and consistent laser transparency across a range of thicknesses and processing conditions of the part to achieve consistent weld strengths.
Several methods have been investigated to increase the NIR laser transparency of compositions based on partially crystalline polyesters. One approach is to blend the partially crystalline polyester with an amorphous resin such as polycarbonate or polyestercarbonate. Such compositions are disclosed, for example, in U.S. Pat. No. 7,396,428 B2 to Matsushima et al. and U.S. Pat. No. 8,052,830 B2 to Sakata et al., and U.S. Patent Application Publication No. US 2011/0256406 A1 of Farrell et al. However, further improvements in NIR laser transparency are desired.
An alternative approach to increase NIR laser transparency is to speed up the rate of crystallization of the composition using a chemical nucleant. This can occur by chemical reaction between the nucleating agent and polymeric end groups of the partially crystalline polyester to produce ionic end groups that enhance the rate of crystallization. Such compositions are disclosed, for example, in U.S. Pat. No. 8,318,843 B2 to Benten et al., and U.S. Patent Application Publication No. US 2011/0306707 A1 of Benten et al. The addition of such chemical nucleants, however, can lower the molecular weight of the partially crystalline polyester and lead to unstable melt viscosity. Additionally, such chemical nucleants can substantially degrade the amorphous resin, causing unstable melt viscosities and other undesirable defects such as splay and jetting (deformations due to turbulent flow).
There remains a need for increased NIR laser transparency in compositions based on partially crystalline polyesters.